50 FLORIDA REEFS. 



such an animal fully expanded, and found it to be a true Acaleph. It is 

 exceedingly difficult to obtain a view of them in this state, for, at any 

 approach, they draw themselves in, and remain closed to all investigation. * 



With these branching corals the reef reaches the level of high water, 

 bevond which, as I hove said, there can be no further growth, for want of 

 the action of the fresh sea-water. This dependence upon the vivifying 

 inlluence of the sea accounts for one unfailing feature in the coral walls. 

 They are always abrupt and steep on the seaward side, but have a gentle 

 slope towards the land. This is accounted for by the circumstance that 

 the corals on the outer side or the reef are in immediate contact with 

 the pure ocean-water, while by their growth they partially exclude the 

 inner ones from the same influence, — the rapid growth of the latter 

 being also impeded by any impurity of foreign material washed away 

 from the neighboring shore and mingling with the water that fills the 

 channel between the main-land and the reef Thus the coral reefs, whether 

 built around an island, or along a straight line of coast, or concentric to 

 a rounding shore, are always shelving toward the land, while they are 

 comparatively abrupt and steep toward the sea. This should be remem- 

 bered, for, as we shall see hereafter, it has an important bearing on the 

 question of time as illustrated by coral reefs. 



I have spoken of the budding of corals, by which each one becomes the 

 centre of a cluster; but this is not the only way in which they multiply 

 their kind. They give birth to eggs also, which are carried on the inner 

 edge of their partition-walls, till they drop into the sea, where they float 

 about, little, soft, transparent, pear-shaped bodies (Plate XVI. Figs. 7, 10, 

 and 11), as unlike as possible to the rigid stony structure they are to 

 assume hereafter. In this condition they are covered with vibratile cilia 

 or fringes, that are always in rapid, uninterrupted motion, and by 

 means of which they swim about hi the water. These little germs of the 

 corals, swimming freely about during their earliest phases of life, contmue 

 the growth of the reef, those that prosper at shallower depths coming in 

 at the various heights where their predecessors die out; otherwise it would 

 be impossible to understand how this variety of building material, as it 

 were, is introduced wherever it is needed. This point, formerly a puzzle 

 to naturalists, has become quite clear since it has been found that myriads 



* See also Agassiz, L., Cont. Nut. Hist. U. S., Vol. III. PI. XV. and II. N. Moseley, Phil. Trans. R. S., 

 Vol. CLXVII. Part 1. 



